Sunlight. Stress. Hot beverages. Doctors have pinpointed many of the triggers that make rosacea worse in people who have it. And anyone affected by the condition is familiar with the discomfort and distress rosacea on the face causes.

But what medical experts haven’t yet pinpointed is this: How is rosacea caused? In other words, they don’t truly know why these triggers lead to redness, thickened skin, bumps, and acne-like breakouts in some people but not others.

“Like the majority of other dermatologic conditions, it’s likely multifactorial,” says a dermatologist and Mohs surgeon in New York. “It’s hard to point the finger at one thing.” Here, some of the leading theories about rosacea causes.

Your genes

Anyone’s skin can briefly flush or turn red—but in people with rosacea, blood vessels become enlarged, causing redness and other symptoms that linger rather than resolve, Dr. Torbeck says.

Rosacea tends to run in families, so experts believe there may be a genetic link—some tweak in your genetic code that boosts your odds of being sensitive to these triggers.

People of northern European descent—especially fair-skinned people with English, Irish, and Scottish heritage—are more often affected, says dermatologist , medical director of Chicago Cosmetic Surgery and Dermatology.

A dysfunctional immune system

Your immune system is supposed to protect your body from harmful invaders. But one theory of how rosacea is caused holds that something goes awry along the way.

When susceptible people are exposed to skin damage from the sun’s ultraviolet rays or to microorganisms on the skin’s surface, this dysfunction may cause an out-of-proportion immune reaction.

“This inappropriate immune response leads to production of inflammatory factors that cause the symptoms of rosacea,” says John Barbieri, MD, a research fellow in dermatology at the University of Pennsylvania’s Perelman School of Medicine.

For instance, compounds called have been shown to be in the blood of people with rosacea, potentially causing pimples and bumps. —which cause allergies and other sensitivities—may also play a role.

Overgrowth of microorganisms

About those bugs: Another potential contributing factor is a strange, small skin mite called Demodex folliculorum. When viewed through a microscope, “they look a little bit like a little Godzilla,” Dr. Torbeck says.

“They live their whole life cycle in your skin and die," he explains. "Normally it’s no issue, but one of the reasons people with rosacea might be more prone is because they have a higher mite load on their skin.”

Faulty connections between nerves and blood vessels

In addition to malfunctions in the immune system, mishaps in the nervous system may also underlie rosacea. After all, it’s nerves that send signals to blood vessels about when to contract and when to relax. When this messaging mechanism doesn’t work properly, too much blood flow to the skin could cause flushing and redness, Dr. Barbieri says.

Researchers have also found people with rosacea may have neurons use to communicate with each other. This could cause an array of abnormal reactions that trigger pustules, bumps, and redness, among other rosacea symptoms.

Infections...maybe

For a while, many people thought H. pylori—a type of bacteria linked to ulcers and intestinal infections—contributed to rosacea. But plenty of people who have rosacea do not have this infection. “The data is very weak at best,” Dr. Torbeck says, and .

No matter the cause, there’s one thing doctors are certain about: Treatment can help ease symptoms of rosacea and reduce its impact on your life.

So if you’ve noticed redness, bumpiness, pimples, pustules, or the appearance of small blood vessels under your skin, consult a dermatologist. The earlier you receive a diagnosis and therapy, the more likely you can avoid longer-term complications of rosacea, including thickening of the skin and eye problems.